Too bad he used elongated projectiles, probably because of the length of the solenoids. You can see that many of them keyholed.
I swear, someday some smart magnetics cookie is going to figure out a way to impart rotation magnetically.
Otherwise, we're kinda stuck with round balls with a spin imparted to them by friction. You don't have to spin a ball axially to get the benefits --the main deal in spinning the ball is to keep the air from piling up against the front, and then side-slipping off the front in odd directions to push the ball sideways randomly.*
I have a propane-driven Colt Mark IV clone which has a little adjustable doohickey in the barrel for imparting a horizontal spin and that thing is amazingly accurate for a round-ball (BBs) firing gun.
And with round balls it could use shorter solenoids.... and more of them, come to think of it.
The accuracy of spinning round balls was shown in 1742, before rifling was common, by some guy firing round balls through a bent smoothbore barrel and noting that the grouping was much better. You will note that the spin in his case was not axial. (Cf pp 142-145 , "Gunpowder" by Jack Kelly, ISBN 0-465-03718-6. A softcover edition is also available.)
He had bent the barrel by some four inches to show this.
Terry, 230RN
* This effect was not as great with heavy round shot from artillery, since the cannonballs were so much heavier than balls out of a musket, compared to the air they were going through. But it was still there to some extent.